Ofdm signal collision position detection apparatus and ofdm reception device

ABSTRACT

A pilot reception power measuring section  104  measures reception power of a pilot symbol and a data section reception power prediction section  106  predicts reception power of data symbols based on the reception power of the pilot symbol. A power comparison section  107  calculates a difference between this predicted value and the actual reception power of the data symbol, and when the difference is large, a collision position detection section  108  regards the data symbol at the hopping position as colliding with data symbols in other cells. Then, an error correcting decoding section  103  carries out error correcting processing by reducing likelihood of the data symbols detected to be involved in the collision and can thereby improve the error rate characteristic of decoded data.

TECHNICAL FIELD

The present invention relates to an OFDM signal collision positiondetection apparatus, an OFDM reception apparatus and a method therefor,and is suitable for use in an OFDM reception apparatus used in an OFDMsystem based on a frequency hopping scheme, etc.

BACKGROUND ART

In a mobile communication system, an OFDM scheme using frequency hoppingis under study. An OFDM system using frequency hopping uses differenthopping patterns among a plurality of cells so as to carry outcommunications by averaging interference among cells.

That is, when two neighboring cells A, B as shown in FIG. 1 areconsidered, a base station BSA in the cell A and a base station BSB inthe cell B transmit OFDM signals with different hopping patterns. Sincethese hopping patterns are normally determined at random in the cell Aand cell B, there is a possibility that collision may occur by accidenton a certain subcarrier at a certain time point.

This will be explained using FIG. 2. FIG. 2 shows a frequency hoppingOFDM signal transmitted from the base station BSA in the cell A and afrequency hopping OFDM signal transmitted from the base station BSB inthe cell B. One unit on the vertical axis shows a subcarrier and oneunit on the horizontal axis shows a 1-burst period. That is, suppose oneOFDM symbol is arranged in one square in the figure.

As is also clear from FIG. 2, an OFDM signal in the cell A collides withan OFDM signal in the cell B by accident on a certain subcarrier at acertain time point. As shown in FIG. 3, reception quality of a datasymbol placed on the subcarrier at the time of collision deterioratescompared to other data symbols.

Thus, in an OFDM system using frequency hopping, the quality of symbolsaffected by interference from other cells deteriorates, and therefore itis necessary to carry out error correcting processing at the time ofdecoding to correct the data of the symbols whose quality isdeteriorated to correct decoded data.

Here, a normal error correcting code is designed to carry out errorcorrection on assumption that the communication path is affected bywhite Gaussian noise. However, in a system like OFDM using frequencyhopping, its communication path does not show white Gaussian noise butit shows a state in which impulse-like noise is added. For this reason,there is a problem that its error correcting performance deteriorates.

In order to correctly decode the signal affected by this impulse-likenoise, there is also a code such as a Reed-Solomon code which is used tocarry out error correcting processing by regarding symbols of poorreception quality as having been lost. However, even when a Reed-Solomoncode is used, it is necessary to correctly notify the error correctingdecoding section of symbols of poor reception quality.

In order to obtain decoded data having a good error rate characteristicfrom such a frequency hopping OFDM signal, it is necessary to correctlydetect symbols involved in a collision. As one of such methods fordetecting symbols, use of the method disclosed in the UnexaminedJapanese Patent Publication No. HEI 11-252040 can be considered.

The technology described in the above Publication detects a subcarrieraffected by interference by monitoring the state of a pilot signalplaced on a specific subcarrier of an OFDM signal. Then, by carrying outweighting processing such as loss correction during error correctingprocessing according to the detection result, the above technologyobtains decoded data with a good error rate characteristic even wheninterference occurs.

However, even if an attempt is made to detect the collision position ona frequency-hopped OFDM signal using the conventional interferencedetection apparatus described in the above Publication, it is notpossible to detect the position of a data symbol on which the collisionhas occurred. This is because while a pilot signal is a signal placed ona predetermined subcarrier at a predetermined timing, a collision of adata symbol occurs on a subcarrier and at a timing which cannot bepredicted from the pilot signal.

Therefore, it is possible to consider a method for detecting a symbol ofpoor quality as a symbol on which a collision has occurred by directlymeasuring the reception quality of a data symbol. However, since thedata symbol is not a known signal, it is not possible to measure thereception quality (e.g., SIR (Signal to Interference Ratio)) of the datasymbol.

Thus, the conventional OFDM system using frequency hopping cannotcorrectly detect a data symbol on which a collision has occurred. As aresult, there is a problem that the error rate characteristic of decodeddata deteriorates.

Furthermore, this problem is not limited to the OFDM system usingfrequency hopping, but can also occur, for example, in an OFDM systemhaving a frequency scheduler. That is, this type of OFDM system isdesigned to measure the channel quality of each subcarrier in each cell,place a data symbol on a subcarrier of good channel quality and transmitit. But in such a case, there is also a possibility that a collision mayoccur on a certain subcarrier between neighboring cells and causedeterioration of the error rate characteristic of decoded data.

DISCLOSURE OF INVENTION

It is an object of the present invention to provide an OFDM signalcollision position detection apparatus and a method therefor capable ofaccurately detecting the positions of data symbols colliding with eachother between a plurality of cells and further provide an OFDM receptionapparatus and a method therefor capable of obtaining decoded data withan improved error rate characteristic using detected collision positioninformation.

This object can be attained by comparing reception power of a datasymbol predicted from reception power of a pilot symbol with the actualreception power of the data symbol for each subcarrier and every burstperiod and regarding, when the reception power of the data symbol ischanged from the reception power predicted from the reception power ofthe pilot symbol, the data symbol as being affected by interference(that is, there is a collision between the neighboring cells). Then, bynotifying an error correcting decoding section of the detected datasymbol, it is possible to improve the error rate characteristic ofdecoded data.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates neighboring cells;

FIG. 2 illustrates a collision between data symbols of frequency hoppingOFDM signals;

FIG. 3 illustrates deterioration of quality of a data symbol caused by acollision;

FIG. 4 is a block diagram showing the configuration of an OFDM receptionapparatus according to Embodiment 1 of the present invention;

FIG. 5 illustrates a threshold X1 for detecting a data symbol whosereception power has increased due to a collision;

FIG. 6 illustrates a threshold X2 for detecting a data symbol whosereception power has decreased due to a collision;

FIG. 7 illustrates a phase relationship between cells when receptionpower increases due to a collision;

FIG. 8 illustrates a phase relationship between cells when receptionpower decreases due to a collision;

FIG. 9 is a block diagram showing the configuration of an OFDM receptionapparatus according to Embodiment 2 of the present invention;

FIG. 10 is a block diagram showing the configuration of an OFDMreception apparatus according to Embodiment 3 of the present invention;

FIG. 11 illustrates a characteristic curve diagram showing arelationship between a threshold and error rate for each SIR; and

FIG. 12 is a block diagram showing the configuration of an OFDMreception apparatus according to Embodiment 4.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the attached drawings, embodiments of the presentinvention will be explained in detail below.

EMBODIMENT 1

FIG. 4 is a block diagram showing the configuration of an OFDM receptionapparatus according to Embodiment 1 of the present invention. The OFDMreception apparatus 100 receives a frequency-hopped OFDM signal by anantenna AN, applies radio reception processing such as down-conversionand analog/digital conversion, etc., by a reception radio (reception RF)section 101, and then sends the processed signal to a fast Fouriertransform circuit (FFT) 102. The FFT 102 obtains a symbol superimposedon each subcarrier by applying fast Fourier transform processing to theinput signal and sends the symbol to an error correcting decodingsection 103, a pilot reception power measuring section 104 and a datasection reception power measuring section 105.

The pilot reception power measuring section 104 measures reception powerof a pilot symbol superimposed on a predetermined subcarrier at apredetermined timing to thereby measure reception power of the pilotsymbol of each subcarrier. Unlike the data symbol, the pilot symbol isplaced at a position where no collision occurs between neighboringcells, and therefore the pilot reception power measuring section 104measures reception power not affected by interference betweenneighboring cells.

A data section reception power prediction section 106 predicts receptionpower of the data symbol not affected by interference (collision) basedon information on the power ratio between the pilot symbol and datasymbol and sends the prediction result to a power comparison section107. Here, a pilot symbol is sent with greater power than that of a datasymbol and the power ratio is a fixed value. The OFDM receptionapparatus 100 stores the power ratio information in memory (not shown).For example, when the power ratio between the pilot symbol and datasymbol is 2:1, the data section reception power prediction section 106multiplies the measurement result of the pilot reception power measuringsection 104 by ½ and outputs the multiplication result as the predictedreception power of the data symbol.

The data section reception power measuring section 105 measures thereception power of the actually received data symbol for each subcarrierand every burst period and sends the measurement result to the powercomparison section 107.

The power comparison section 107 compares the reception power of thedata symbol predicted by the data section reception power predictionsection 106 with the actual reception power of the data symbol measuredby the data section reception power measuring section 105 for eachsubcarrier and sends the comparison result to a collision positiondetection section 108. The power comparison section 107 actuallycalculates a difference between the prediction result by the datasection reception power prediction section 106 and the measurementresult by the data section reception power measuring section 105 foreach corresponding subcarrier and every burst period and sends thedifference to the collision position detection section 108.

The collision position detection section 108 detects a position wherethere is a large variation in the actually measured reception power ofthe data signal with respect to the reception power of the data signalpredicted from the pilot signal (that is, position where the differencevalue is large) based on the difference value from the power comparisonsection 107 and thereby detects the positions of data symbols collidingwith each other between a plurality of cells.

The collision position detection section 108 carries out detectionprocessing only at a position indicated by hopping pattern information(subcarrier and burst period: e.g., position of the cell A in FIG. 2).This hopping pattern is notified beforehand from the base station in thecell to which the own station belongs. When the collision positiondetection section 108 detects the collision position of the data symbol,it sends the collision position to the error correcting decoding section103 as collision position information.

In this embodiment, the collision position detection section 108 makes athreshold decision using two thresholds X1, X2 to detect the collisionposition. Here, as shown in FIG. 5, the first threshold X1 is athreshold in a positive direction taking into account an increase ofpower due to a collision, while the second threshold X2 is a thresholdin a negative direction taking into account the power decrease due to acollision as shown in FIG. 6.

As shown in FIG. 7, when another cell signal having a similar phasecollides with the own cell signal, the actually measured reception powerof the data symbol is greater than the reception power of the datasymbol predicted from the pilot symbol. That is, in such a case, thereception power increases due to the collision. On the other hand, asshown in FIG. 8, when another cell signal having a substantiallyopposite phase collides with the own cell signal, the actually measuredreception power of the data symbol is smaller than the reception powerof the data symbol predicted from the pilot symbol. That is, in such ascase, the reception power decreases due to the collision.

Considering this, according to this embodiment, when the actuallymeasured reception power of the data symbol is greater than the firstthreshold X1 in a positive direction with respect to the data symbolreception power predicted from the pilot symbol or smaller than thesecond threshold X2 in a negative direction, the position detectionsection 108 detects the case as a collision. This makes it possible todetect a collision between any data symbols without exceptionirrespective of the phases of data symbols involved in the collision.

The error correcting decoding section 103 obtains decoded data bycarrying out decoding processing while applying error correctingprocessing to the data symbols. In this case, the error correctingdecoding section 103 applies error correcting decoding processing byreducing the likelihood of the data symbols at the position at which thecollision is detected by the collision position detection section 108.For example, when error correction using a Reed-Solomon code is carriedout, the error correcting decoding section 103 carries out errorcorrecting processing by regarding data symbols detected to be involvedin a collision as a loss. This makes it possible to remove impulse-likenoise due to the collision of data symbols from the error correctingprocessing and thereby improve the error rate characteristic of thedecoded data.

According to the above described configuration, reception power of adata symbol is predicted based on reception power of a pilot symbol, andwhen there is a large difference between this predicted value and theactual reception power of the data symbol, the data symbol at thehopping position is regarded as colliding with a data symbol in anothercell and it is thereby possible to accurately detect data symbolscolliding with each other between a plurality of cells.

Furthermore, the data symbols involved in the collision are notified tothe error correcting decoding section 103 and the error correctingdecoding section 103 carries out error correcting processing with thelikelihood of the notified data symbols reduced, and it is therebypossible to improve the error rate characteristic of the decoded data.

This embodiment has described the case where first and second thresholdsX1, X2 are used and a collision is detected when the measured receptionpower of the data symbol is greater than the first threshold X1 in apositive direction with respect to the predicted reception power of thedata symbol or smaller than the second threshold X2 in a negativedirection, but the present invention is not limited to this and it isalso possible to simply determine a threshold x relative to which anactually measured value is regarded as having changed from a predictedvalue and consider that a collision has occurred when the change isgreater than x [dB]. The same will apply to Embodiments 2 to 4 whichwill be described later.

Furthermore, this embodiment has described the case where information ondata symbols involved in a collision detected by the collision positiondetection section 108 is notified to the error correcting decodingsection 103 so as to improve the error rate characteristic at the errorcorrecting decoding section 103, but the use of the information on thedata symbols involved in a collision detected by the collision positiondetection section 108 is not limited to this. The data symbolinformation can lso be used, for example, as information for aretransmission request and can be used for various purposes.

That is, it is possible to accurately detect the positions of datasymbols colliding with each other using the collision position detectionapparatus including the pilot reception power measuring section 104 as aknown signal measuring section, the data section reception powermeasuring section 105 as a data signal measuring section, the datasection reception power prediction section 106 as a data signalprediction section, the power comparison section 107 as a powercomparison section and the collision position detection section 108 as acollision position detection section.

Furthermore, this embodiment has described the case where an OFDM systemusing a frequency hopping scheme is applied, but the present inventionis not limited to this and is widely applicable to an OFDM system suchas an OFDM system having a frequency scheduler in which there is apossibility that subcarriers on which data symbols are placed maycollide with each other between a plurality of cells.

EMBODIMENT 2

FIG. 9 in which components corresponding to those in FIG. 4 are assignedthe same reference numerals shows the configuration of an OFDM receptionapparatus according to Embodiment 2. The OFDM reception apparatus 200has the same configuration as that of the OFDM reception apparatus 100according to Embodiment 1 except that it includes an error ratecalculation section 201 that calculates an error rate of decoded dataand a threshold control section 202 that changes a threshold at acollision position detection section 108 according to the error rate.

The error rate calculation section 201 calculates an error rate ofdecoded data output from an error correcting decoding section 103 andsends the calculated error rate to the threshold control section 202.The threshold control section 202 changes a threshold used in thecollision position detection section 108 according to the error rate.This allows the OFDM reception apparatus 200 according to thisembodiment to use a threshold which optimizes the error ratecharacteristic.

More specifically, the threshold control section 202 adaptively changesthe threshold while monitoring the error rate to thereby make thethreshold converge to an optimal value which optimizes the error ratecharacteristic and supplies the threshold to the collision positiondetection section 108.

Here, a threshold x used in the collision position detection section 108includes a value which optimizes the performance (error ratecharacteristic). This is because when the threshold x is too large, thenumber of data symbols which are regarded as being involved in acollision decreases, the effect of improving the error correctingperformance with respect to a loss decreases, and therefore theperformance deteriorates. On the contrary, when the threshold x is toosmall, the number of data symbols which are regarded as being involvedin a collision increases, substantially all symbols are regarded ashaving been lost, and therefore the performance deteriorates.

In addition to the configuration of Embodiment 1, this embodiment isprovided with the threshold control section 202 that controls thethreshold used in the collision position detection section 108 to anoptimal value, and can thereby realize the OFDM reception apparatus 200capable of further improving the error rate characteristic of decodeddata in addition to the effect of Embodiment 1.

EMBODIMENT 3

FIG. 10 in which components corresponding to those in FIG. 4 areassigned the same reference numerals shows the configuration of an OFDMreception apparatus according to Embodiment 3. The OFDM receptionapparatus 300 has the same configuration as the OFDM reception apparatus100 of Embodiment 1 except that it includes a reception average SIRdetection section 301 that detects an average SIR (Signal toInterference Ratio) of an OFDM signal received as a reception qualitydetection section and a table section 302 that stores data of thresholdsassociated with reception quality and outputs the threshold datacorresponding to the detected reception quality to a collision positiondetection section 108.

The reception average SIR detection section 301 detects an SIR based ona pilot symbol and calculates an average value of the SIR for, forexample, a 1-slot period to detect reception quality of a received OFDMsignal. The detected reception quality is sent to the table section 302.

As shown in, for example, FIG. 11, the table section 302 stores optimalthreshold data th1, th2 for the SIR for each predetermined SIR (FIG. 11shows only SIRs of 15 [dB] and 10 [dB] for simplicity of explanation).The table section 302 sends the optimal threshold th1, th2 for the SIRclosest to the SIR output from the reception average SIR detectionsection 301 to the collision position detection section 108.

In a simple explanation of FIG. 11, when the SIR is better (15 [dB] inthe figure), the variation in the received signal due to interferencemust be smaller. In this case, the optimal threshold th1 has arelatively small value. On the contrary, when the SIR is bad (10 [dB] inthe figure), the variation in the received signal due to interferenceincreases. In this case, the optimal threshold th2 has a relativelylarge value.

Noting that the optimal threshold used in the collision positiondetection section 108 varies depending on the reception quality, thisembodiment provides the table section 302 storing optimal thresholdsaccording to the reception quality.

Here, as is also evident from FIG. 11, when the threshold is decreasedfrom the optimal threshold th1, th2, the number of data symbols detectedto be involved in a collision increases and when the threshold isincreased from the optimal threshold th1, th2, the number of datasymbols detected to be involved in a collision decreases and in bothcases, the error rate characteristic deteriorates. In this regard, thecase where the threshold is set to infinity corresponds to conventionalgeneral processing when no consideration is given to a collision.

Thus, in addition to the configuration of Embodiment 1, this embodimentis provided with the reception average SIR detection section 301 thatdetects reception quality and the table section 302 storing optimalthresholds according to the reception quality so that the collisionposition detection section 108 uses an optimal threshold output from thetable section 302 according to the reception quality, and can therebyrealize the OFDM reception apparatus 300 capable of further improvingthe error rate characteristic of decoded data in addition to the effectof Embodiment 1.

EMBODIMENT 4

FIG. 12 in which components corresponding to those in FIG. 4 areassigned the same reference numerals shows the configuration of an OFDMreception apparatus according to Embodiment 4. The OFDM receptionapparatus 400 has the same configuration as the OFDM reception apparatus100 of Embodiment 1 except that it includes a reception average SIRdetection section 401 that detects an average SIR (Signal toInterference Ratio) of a received OFDM signal as a reception qualitydetection section, a table creation section 402, an error ratecalculation section 403 and a threshold control section 404.

The reception average SIR detection section 401 detects an SIR based ona pilot symbol, calculates an average of the SIR for, for example, a1-slot period and thereby detects reception quality of the received OFDMsignal. The detected reception quality is sent to the table creationsection 402.

The table creation section 402 creates a lookup table showing arelationship between a threshold input from the threshold controlsection 404 and an error rate calculated by the error rate calculationsection 403 for each reception quality (average SIR in this embodiment)Furthermore, the table creation section 402 supplies data correspondingto the SIR detected by the reception average SIR detection section 401out of the data in the created lookup table to the threshold controlsection 404.

The threshold control section 404 looks up reference data input from thetable creation section 402, finds an optimal threshold and sends it to acollision position detection section 108. More specifically, the tablecreation section 402 collects data showing a characteristic curve foreach reception quality shown in FIG. 11 and the threshold controlsection 404 receives data corresponding to the reception quality, findsoptimal thresholds th1, th2 and sends them to the collision positiondetection section 108.

That is, what is different from Embodiment 3 is that Embodiment 3 storesan optimal threshold for each reception quality in the table section 302beforehand, whereas in this embodiment the table creation section 402updates data one by one and creates the characteristic curve as shown inFIG. 11. This makes it possible to set more appropriate optimalthresholds from the relationship between the actual threshold and errorrate and further improve the error rate characteristic.

Thus, in addition to the configuration of Embodiment 1, this embodimentprovides the reception average SIR detection section 401 that detectsreception quality, the threshold control section 404, the error ratecalculation section 403 and the table creation section 402 that createsa lookup table showing the relationship between the threshold and theerror rate when the threshold controlled by the threshold controlsection 404 is used for each reception quality, and looks up the lookuptable updated at any time through the threshold control section 404 andsets an optimal threshold at the collision position detection section108, and can thereby realize an OFDM reception apparatus capable offurther improving the error rate characteristic compared to Embodiment3.

Foregoing Embodiments 3, 4 have described the case where an average SIRis detected as reception quality, but it is also possible to detect, forexample, CIR (Carrier to Interference Ratio), etc., and the receptionquality to be detected is not limited to SIR. Furthermore, the tablesection 302 of Embodiment 3 stores optimal thresholds corresponding toSIRs, but the present invention is not limited to this and the point isto store an optimal threshold corresponding to reception quality.Likewise, the table creation section 402 of Embodiment 4 has describedthe case where a table which associates a threshold with an error ratefor each predetermined SIR is created, but SIR is not the only factorand the point is to create a table which associates a threshold with anerror rate for each reception quality.

The present invention is not limited to the above described embodimentsbut can be implemented modified in various ways.

An aspect of the OFDM signal collision position detection apparatus ofthe present invention is an OFDM signal collision position detectionapparatus that detects collision positions of OFDM signals transmittedfrom a plurality of cells, comprising a known signal measuring sectionthat measures reception power of a known signal, a data signal measuringsection that measures reception power of a data signal, a data signalprediction section that predicts reception power of the data signalbased on the measured reception power of the known signal, a powercomparison section that compares the reception power of the data signalpredicted by the data signal prediction section with the reception powerof the data signal measured by the data signal measuring section foreach subcarrier and a collision position detection section that detectspositions of data symbols colliding with each other between a pluralityof cells by detecting positions where there is a large variation of themeasured reception power of the data signal with respect to thepredicted reception power of the data signal based on the comparisonresult.

According to this configuration, the known signal measuring sectionmeasures reception power of a known signal not affected by interferencecaused by a collision between a plurality of cells based on the knownsignal arranged so as to prevent any collision between the plurality ofcells, while the data signal prediction section predicts reception powerof a data signal when no collision occurs based on the reception powerof this known signal. The power comparison section compares the actualreception power of the data signal with the reception power of the datasignal when no collision occurs for each subcarrier and the collisionposition detection section detects the position of a data symbol havinga large variation (difference) as the collision position. As a result,it is possible to accurately detect positions of data symbols collidingwith each other between a plurality of cells, that is, subcarriers andtime point.

In another aspect of the OFDM signal collision position detectionapparatus of the present invention, the collision position detectionsection detects a collision when measured reception power of a datasignal is greater than a first threshold in a positive direction withrespect to predicted reception power of the data signal or smaller thana second threshold in a negative direction.

According to this configuration, when the phases of data symbolsinvolved in a collision are similar, the collision is detected based ona first threshold and when the phases of data symbols involved in thecollision are not similar, the collision is detected based on a secondthreshold, and therefore it is possible to detect any collision betweendata symbols without exception irrespective of the phases of datasymbols involved in the collision.

A further aspect of the OFDM reception apparatus of the presentinvention is an OFDM reception apparatus that receives and demodulatesan OFDM signal, comprising a known signal measuring section thatmeasures reception power of a known signal, a data signal measuringsection that measures reception power of a data signal, a data signalprediction section that predicts reception power of the data signalbased on the measured reception power of the known signal, a powercomparison section that compares the reception power of the data signalpredicted by the data signal prediction section with the reception powerof the data signal measured by the data signal measuring section foreach subcarrier, a collision position detection section that detectspositions of data symbols colliding with each other between a pluralityof cells by detecting positions where there is a large variation of themeasured reception power of the data signal with respect to thepredicted reception power of the data signal based on the comparisonresult and an error correcting decoding section that applies errorcorrecting decoding processing to the received OFDM signal by reducinglikelihood of data symbols at positions at which the collision positiondetection section has detected the collision.

According to this configuration, the known signal measuring section,data signal measuring section, data signal prediction section, powercomparison section and collision position detection section accuratelydetect positions of data symbols (subcarriers, time point) involved inthe collision between a plurality of cells. Furthermore, the errorcorrecting decoding section carries out error correcting processing byreducing likelihood of data symbols detected to be involved in thecollision, and can thereby remove impulse-like noise due to thecollision of data symbols from the error correcting processing andimprove the error rate characteristic of decoded data.

In a still further aspect of the OFDM reception apparatus of the presentinvention, the collision position detection section uses a threshold fordetecting a position where the above described variation is large andfurther comprises, in addition to the above described configuration, anerror rate calculation section that calculates an error rate of decodeddata obtained by the error correcting decoding section and a thresholdcontrol section that changes a threshold at the collision positiondetection section according to the error rate calculation result.

According to this configuration, the number of data symbols which areregarded as being involved in the collision increases when the thresholdcontrol section decreases the threshold, whereas the number of datasymbols which are regarded as being involved in the collision decreaseswhen the threshold control section increases the threshold. This numberof data symbols regarded as being involved in the collision has a greateffect on the error rate. In consideration of this, the thresholdcontrol section adaptively changes the threshold according to the errorrate calculation result, and can thereby detect data symbols involved inthe collision using an optimal threshold with which an optimal errorrate characteristic is obtained.

In a still further aspect of the OFDM reception apparatus of the presentinvention, the collision position detection section uses a threshold indetecting positions where the above described variation is large andfurther comprises, in addition to the above described configuration, areception quality detection section that detects reception quality ofthe received OFDM signal and a table that stores data of thresholdsassociated with the reception quality and outputs threshold datacorresponding to the detected reception quality to the collisionposition detection section.

According to this configuration, an optimal threshold corresponding toreception quality is supplied to the collision position detectionsection from the table section, and therefore the collision positiondetection section can detect the collision position more accurately andthereby further improve the error rate characteristic as a result.

In a still further aspect of the OFDM reception apparatus of the presentinvention, the collision position detection section uses a threshold indetecting positions where the above described variation is large andfurther comprises, in addition to the above described configuration, anerror rate calculation section that calculates an error rate of decodeddata obtained from the error correcting decoding section, a receptionquality detection section that detects reception quality of the receivedOFDM signal, a threshold control section that controls a threshold inthe collision position detection section, a table creation section thatcreates a lookup table showing a relationship between a threshold and anerror rate when the threshold controlled by the threshold controlsection is used for each reception quality, wherein the thresholdcontrol section sets an optimal threshold in the collision positiondetection section with reference to the lookup table.

According to this configuration, the table creation section updates andcreates a lookup table indicating the relationship between a thresholdand an error rate when the threshold is used at any time for eachreception quality and the threshold control section sets an optimalthreshold in the collision position detection section with reference tothe lookup table updated at any time, and therefore a more accurateoptimal threshold is set as the threshold used by the collision positiondetection section. As a result, the error rate characteristic is furtherimproved.

As described above, by comparing reception power of a data signalpredicted from reception power of a known signal with the actualreception power of the data signal for each subcarrier and every burstperiod and regarding, when the reception power of the data signal ischanged from the reception power predicted from reception power of apilot signal, the data signal as being involved in a collision between aplurality of cells, the present invention can realize a collisionposition detection apparatus capable of accurately detecting positionsof data signals colliding with each other between a plurality of cells.

Furthermore, data symbols detected by the collision position detectionapparatus are notified to the error correcting decoding section and theerror correcting decoding section carries out error correcting decodingprocessing by reducing likelihood of data symbols at positions where thecollision position detection section has detected the collision, andtherefore it is possible to realize an OFDM reception apparatus capableof improving the error rate characteristic of decoded data.

This application is based on the Japanese Patent Application No.2003-23747 filed on Jan. 31, 2003, entire content of which is expresslyincorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention is suitable for use in, for example, a mobilecommunication terminal.

1. An OFDM signal collision position detection apparatus that detectscollision positions of OFDM signals transmitted from a plurality ofcells, comprising: a known signal measuring section that measuresreception power of a known signal; a data signal measuring section thatmeasures reception power of a data signal; a data signal predictionsection that predicts reception power of the data signal based on themeasured reception power of the known signal; a power comparison sectionthat compares the reception power of the data signal predicted by saiddata signal prediction section with the reception power of the datasignal measured by said data signal measuring section for eachsubcarrier; and a collision position detection section that detectspositions of data symbols colliding with each other between a pluralityof cells by detecting positions where there is a large variation of saidmeasured reception power of the data signal with respect to saidpredicted reception power of the data signal based on the comparisonresult obtained by said power comparison section.
 2. The OFDM signalcollision position detection apparatus according to claim 1, whereinsaid collision position detection section detects a collision when saidmeasured reception power of the data signal is greater than a firstthreshold in a positive direction with respect to said predictedreception power of the data signal or smaller than a second threshold ina negative direction.
 3. An OFDM reception apparatus that receives anddemodulates an OFDM signal, comprising: a known signal measuring sectionthat measures reception power of a known signal; a data signal measuringsection that measures reception power of a data signal; a data signalprediction section that predicts reception power of the data signalbased on the measured reception power of the known signal; a powercomparison section that compares the reception power of the data signalpredicted by said data signal prediction section with the receptionpower of the data signal measured by said data signal measuring sectionfor each subcarrier; a collision position detection section that detectspositions of data symbols colliding with each other between a pluralityof cells by detecting positions where there is a large variation of saidmeasured reception power of the data signal with respect to saidpredicted reception power of the data signal based on the comparisonresult obtained by said power comparison section; and an errorcorrecting decoding section that applies error correcting decodingprocessing to the received OFDM signal by reducing likelihood of datasymbols at positions at which said collision position detection sectionhas detected the collision.
 4. The OFDM reception apparatus according toclaim 3, wherein said collision position detection section uses athreshold for detecting a position where said variation is large, andsaid OFDM reception apparatus further comprises: an error ratecalculation section that calculates an error rate of decoded dataobtained by said error correcting decoding section; and a thresholdcontrol section that changes said threshold at said collision positiondetection section according to the error rate calculation result.
 5. TheOFDM reception apparatus according to claim 3, wherein said collisionposition detection section uses a threshold in detecting positions wheresaid variation is large, and said OFDM reception apparatus furthercomprises: a reception quality detection section that detects receptionquality of the received OFDM signal; and a table that stores data ofthresholds associated with the reception quality and outputs saidthreshold data corresponding to said detected reception quality to saidcollision position detection section.
 6. The OFDM reception apparatusaccording to claim 3, wherein said collision position detection sectionuses a threshold in detecting positions where said variation is large,and said OFDM reception apparatus further comprises: an error ratecalculation section that calculates an error rate of decoded dataobtained from said error correcting decoding section; a receptionquality detection section that detects reception quality of the receivedOFDM signal; a threshold control section that controls a threshold insaid collision position detection section; and a table creation sectionthat creates a lookup table showing a relationship between saidthreshold and an error rate when the threshold controlled by saidthreshold control section is used for each reception quality, whereinsaid threshold control section sets an optimal threshold in saidcollision position detection section with reference to said lookuptable.
 7. An OFDM signal collision position detection method comprising:a step of predicting reception power of a data signal from receptionpower of a known signal; and a step of comparing reception power of saidpredicted data signal with the actually measured reception power of thedata signal for each subcarrier and every burst period and regarding,when the reception power of the data signal is changed from thereception power predicted from the reception power of the known signal,the data signal as being involved in a collision between a plurality ofcells and thereby detecting positions of data symbols colliding witheach other between a plurality of cells.
 8. An OFDM reception method,comprising: a step of predicting reception power of a data signal fromreception power of a known signal; a step of comparing reception powerof said predicted data signal with the actually measured reception powerof the data signal for each subcarrier and every burst period andregarding, when the reception power of the data signal is changed fromthe reception power predicted from the reception power of the knownsignal, the data signal as being involved in a collision between aplurality of cells and thereby detecting positions of data symbolscolliding with each other between a plurality of cells; and a step ofapplying error correcting decoding processing by reducing likelihood ofdata symbols at positions where the collision is detected.